1. Field of Invention
This invention relates to equipment used in connection with the cementing of casing strings in earthen boreholes. More particularly, this invention relates to wiper plugs used in the cementing process.
2. Description of Prior Art
In the field of drilling earthen boreholes or “wells,” particularly wells for oil and gas production, each section of open hole (that is, the hole drilled in the earth) is generally cased off by a length of iron or steel casing placed into the borehole. This length of casing is commonly referred to as a “casing string.” Some of the purposes of casing are to maintain the structure of the sediment surrounding the hole, as well as to prevent contamination of any nearby oil or water structure. Other purposes relate to the containment of drilling fluids needed to control subsurface pressures. At the very bottom of the casing string is usually a “float shoe,” and one or more (but generally no more than two or three) joints up (commonly called “shoe joints”) is a “float collar.” Both the float shoe and float collar usually contain one-way or check valves, which permit pumping of fluids (including drilling fluids and cement) down through the float collar and float shoe, yet prevent fluid flow in the reverse direction, or back into the interior of the casing string.
Typically, after the casing string is lowered into the hole, it is cemented in place. A typical cementing procedure is to insert a first or bottom plug into the casing string. One of the purposes of the bottom plug is to wipe the inner wall of the casing string substantially free from any debris, and any drilling mud adhering to the inner casing wall, that may potentially impede the cementing process. Yet another purpose is to separate the cement slurry from the drilling mud preceding it. The bottom plug is pumped downhole by the cement slurry. Following the cement slurry is usually a second wiper plug, called the top plug. Thereafter, the two plugs with the cement volume therebetween are pumped downhole by a volume of drilling fluid or mud. The top plug also serves as a barrier between the cement slurry and the drilling mud used as the displacing fluid.
Once the bottom plug reaches the float collar, pumping pressure is increased until the diaphragm in the bottom plug ruptures, allowing the cement to flow through the plug, then through the float collar and float shoe, and outward and upward into the annulus between the casing and the open borehole and/or previous casing string. Pumping continues until the top plug reaches the bottom plug (which is lodged against the float shoe), at which point an increase in the pump pressure shows that the top plug has “bumped.”
Problems arise where drilling is to continue beyond the casing string depth. The initial “drillout” must drill through both wiper plugs, the float equipment, and the cement in the shoe joint or joints. A potential problem is that one or both of the wiper plugs, which as described earlier have “landed” on the float collar (or float shoe, if no float collar has been run), spin or rotate along with the rotary drill bit, rather than remain rotationally locked in place for easy drillup. Obviously, as long as the plug or plugs spin along with the bit, little or no progress in drilling therethrough can be made, and in some instances much time, and consequently money, is lost. The problem, then, is how to keep the plugs from spinning beneath the drill bit during the drillout procedure.
To combat this problem, prior art has suggested the use of matching teeth or locks on both the float equipment and the wiper plugs. Generally, this solution requires cement wiper plugs and float equipment that are specially made, one for the other, in order to work. Typically, the upper end of the float collar and the lower and upper end of the bottom plug and the lower end of the top plug are provided with matching teeth, intended to mesh together and rotationally lock the plugs together and lock the plugs to the float equipment. Other solutions involve threaded or J-lock engagements between cement wiper plugs and float equipment.
However, a common drawback to the prior art apparatus is the requirement of matched float equipment and cement wiper plugs and/or additional labor and equipment in order to achieve the rotationally locking functions. While the cementing function can be carried out with whether or not the float equipment and plugs have some sort of matching, meshing teeth or other profiles, it can be readily seen that without the matching aspect, the rotationally locking situation will not be achieved. The requirement of “matched” float equipment and plugs gives rise to increased cost, and the ever-present possibility of mismatched equipment being used in the hectic nature of oilfield work.
Yet another limitation of prior art, matched plugs and float equipment is the possibility of a build-up of debris on the matching or mating components, such as teeth, of the cementing equipment, or a fluid flow-back through the float equipment which would separate the plug from the float equipment and therefore unseat the meshing lock profiles. Such a build-up of debris or fluid flow-back often impedes the mating of the matching components, consequently the cement wiper plugs do not rotationally lock in place.
Yet another attempt seen in the prior art to address this problem involves fixing (by adhesive or other means) an internally splined sleeve within the joint of casing immediately above the float collar, into which the wiper plugs are forced. A drawback to this apparatus is binding of the drill bit when the assembly is drilled up, and the ever-present possibility of an incorrect non-rotating sleeve installation.
Therefore, what is needed is a cement wiper plug that rotationally locks into place, without the need of specialized float equipment to engage teeth or other meshing profiles in the wiper plug for rotationally locking the wiper plugs, and that does not pose issues with rotationally binding the drillout assembly.